884 DARYLEH. BUSCHAND DONALD C. JICHA
Inorganic Chemistr?
CONTRIBUTION FROM
THE
EVANS AND MCPHERSON CHEMICAL LABORATORIES, THEOHIOSTATEUNIVERSITY, COLUMBUS, OHIO
Complexes Derived from Strong Field Ligands. XV. The Formation of Trinuclear Complexes by Coordination of Tris-(0-mercaptoethy1amine)-cobalt(II1)to Metal Ions BY DAKYLE H. BUSCH AND DONALD C. JICHA
Received June 13, 1962 Tris-(P-mercaptoethy1amine)-cobalt( 111), CoL3, is diamagnetic and apparently exists in only a single isomeric form. That form is probably the facial or 1-2-3 geometric isomer. CoLI forms highly crystalline compounds of the general formula [M(CoL3)2]X,. The centrally located metal atom M is bound by three mercaptide bridges to each CoL8 unit. These examples establish the existence of triple bridges in mercaptide systems. For M = Ni(II), p e l f = 3.2 B.M
During the course of earlier studies on the complexes of transition metal ions with P-mercaptoethylamines, di- and trinuclear complexes have been isolated and In the larger number of cases, these unusual materials have been derived from planar complexes of the stoichiometry M L (where L represents NH2CHzCH2Sand M is Ni or Pd) by utilizing those substances as coordinating agents and causing them to react with metal ions to form products of the composition [M’(ML2)2]2+.It has been shown that the metal ions in the side and central positions may be permuted a t will by the application of this principle.2 Further, the generality of the reaction provides strong inferential evidence favoring a cis orientation of the mercapto groups in planar mononuclear derivatives of mercaptoamines. This result is in keeping with the orientation of the Torbitals on the central atom. The d,, and d,, orbitals may each interact with the n-orbital of an individual sulfur atom in the cis structure, while a competitive situation exists in n-bonding in the truns planar case. The unsymnietrical nature of p-mercaptoethylamine d s o leads to the possibility of geometrical isomerism in the octahedral case (facial, 1-2-3, and peripheral, 1-2-6). The most readily available example for the search for this type of isomerism should be provided by tris- (P-mercaptoethylamhe)-cobalt(III), COL3. This light blue substance has been prepared by several investig a t o r ~ . The ~ ~ ~material exhibits only a very (1) D. C. Jicha and D. H. Busch, Inovg. Chem., 1, 872 (1962). ( 2 ) D. C. Jicha and D. H. Busch, ibid., 1, 878 (1962). (3) E. Felder, E. Paolio, and U. Tiepolo, Favmaco (Pavia), Ed. Sci., 10, 836 (1955). (4) R. G. Seville and G. Gorin, J . A m . Chem. SOL.,78, 4891, 4893 (1956).
slight solubility in water and the usual organic solvents. It has been prepared by direct oxidation of cobalt(I1) in the presence of the ligand3 and by the displacement of ammonia from C O ( N H ~ ) ~ ~ + . ~ A third synthesis is reported here, in which the P-mercaptoethylamine displaces EDTA4- from the hexadentate complex Co(EDTA)-. Co(EDTA)-
+ N H ~ C H Z C H ~ S--+ H
+
CO(N H z C H K H Z S ) ~ HIEDTA-
The very slight solubility of the product results in gelatinous precipitates by the direct oxidation. The relatively slow displacement reactions permit crystallization of the product as i t forms, thereby leading to a relatively tractable solid of larger crystal size. Magnetic measurements (XM = -55.4 x c.g.s. unit, T = 300”) establish the expected spin paired state of the cobalt atom in CO(NH~CH~CH~S)~. The availability of three distinct synthetic routes for the preparation of the cobalt(II1) complex provides an opportunity to look for geometric isomers although more general separation techniques are not available because of the limited solubility of the substance. The oxidative procedure and the reaction with Co(EDTA)- might be expected to give quite different results in this regard. The former should yield an equilibrium mixture of isomers while the latter should be highly stereospecific, with a good probability of producing only a single i ~ o m e r . ~Despite ,~ this situation, the only detectable difference in samples (5) D. H. Busch, D. W. Cooke, K. Swaminathan, and Y. A. Im, “Advances in the Chemistry of the Coordination Compounds,” S. Kirschner, ed., The MacMillian Co., New York, N. Y . , 1961, p. 139. (6) D. H. Busch, K. Swaminathan, and D. W. Cooke, I n o r g . Chem., 1, 260 (1962).
TRIS-(P-MERCAPTOETHYLAMINE)-COBALT(IH) COMPLEXES885
Vol, 1, No. 4,November, 1962
obtained by different procedures is in the state of subdivision of the solid. By analogy with the nickel(I1) and palladium(11) complexes of the form ML2, a cis or facial configuration may be favored for the cobalt(II1) complex, COL3. The facial arrangement of the sulfur atoms of three ligand molecules would favor the formation of *-bonds with the tegelectrons of the central atom. The dxy, d,,, and d,, orbitals are of proper symmetry to form three equivalent mutually orthogonal a-bonds to groups located in Fig. 1.-The proposed structure for [M{Co1I1(L)sI~]Xn ( L = H2NCH2CH2S-): 0 = sulfur; = nitrogen. the 1, 2, and 3 positions in the octahedral structure. In consequence, the facial configuration is most rationally assumed for the cobalt(II1) comvalent electrolyte. The molar conductance of plex. This conclusion receives strong support in 362 ohm-' is consistent with this view. Although the experiments reported below. it must be admitted that the coordination of broThe tendency of the coordinated sulfur atoms in mides might occur in the solid state and that the C O ( N H Z C H ~ C H to ~ Sform ) ~ bridges to other metal rate of aquation might be extremely fast, thereby ions has been investigated and two unusual trinuproducing a tri-univalent electrolyte by coordinaclear compounds have been prepared in high yield tion of solvent molecules in solution, the result is and purity by taking advantage of this proclivity. consistent with the proposal that only sulfur atoms The insoluble complex COL3 dissolves in solutions are bound to the centermost cobalt. The arguof Co(I1) or Ni(I1) ions and also in solutions of ment remains equivocal a t this point. Co(NH&Br2+. In all cases, deep red-brown soluThe product formed by the dissolution of the tions are obtained. The addition of potassium cobalt complex cOL3 in media containing the bromide to solutions prepared by the dissolution nickel(I1) ion offers a parallel but more convincing of cOL3 in aqueous cobalt(I1) chloride results in' It is immediately suggested that the behavior. the crystallization of dark red-brown crystals of } ~ ] B ~ ~ .2Co(NH2CH2CH2S)a Xi2+ -+ the composition [Co'" { C O ~ ~ ~ ( L ) ~Magnetic c.g.s. unit, measurements (XM = -115 X [N~(CO(NH~CHZCH~S)~)~]~+ T = 303") confirm the oxidation state of three for all three cobalt atoms. It is apparent that air nickel-dicobalt complex should behave as a dioxidation of cobalt(I1) to cobalt(II1) occurs rapunivalent electrolyte. The molar conductance of idly and extensively in this system. The displace231 ohm-' is completely consistent with this ment reaction requirement of the hypothesis. Further, if the proposed structure is correct, the nickel(I1) atom Co( bfH8)bBr*+ 2CoLa + in [Ni(CoL3)2]Brz is located in an octahedral site, [CO(COL&]~+ Br5NH3 with perhaps only a weak trigohal component associated with the threefold axis passing through proceeds smoothly, yielding an identical product the centers of the three metal ions. In consec.g.s. unit, T = for which XM = -119 X quence, the precepts of modern ligand field theory 303". suggest that the nickel(I1) atom may well exhibit An interesting interrelation exists between the a normal spin-free ground state.' This expectastructures of CoL3 and the trinuclear species tion is fully supported by the observed magnetic [ C O ( C O L ~ ) ~ ]In ~ +the . event that the structure of moment of 3.23 B.M. for the bromide salt. The CoLs involves mutual cis orientation of all three fact must be emphasized that this result is not sulfur atoms, as suggested above, each mole of this complex may form three sulfur bridges to the explainable in terms of the weakened donor ability of a sulfur atom which might occur as a result of unique (third) cobalt atom. Thus the sulfur atoms prior coordination to another metal atom. For from two moles of cOL3 can occupy all six coordination positions about the third cobalt atom, example, in the compounds [Ni(NiL&]Xz, [Nias shown in Fig. 1. It follows that the anhydrous (PdL2)2]Xz, and [Pd(NiL&]X*, the nickel(I1) salt [Co(CoL&]Br~should dissolve without reac(7) C . H. Ballhausen and A. D Liehr, J. A m . Chem. Sod.. 81 tion to produce a solution containing a tri-uni538 (1959).
+
+
+
+
88.6 DARYLE H. BUSCHAND DONALD C. JICHA atom is best able to form a square planar array of donor atoms and the expected diamagnetism is observed.'Z2 It is, of course, not unusual to observe spin-free nickel(I1) in complexes with strong donorsIs although the inorganic chemist is just beginning to realize that the stereochemical limitations and efficacies of various ligands may play as significant a role in establishing the multiplicity of the ground state as does the ligand field strength. The existence and properties of trinuclear materials of the composition [M(CoL&]X, is best explained on the basis of the structure given in Fig. 1. These substances serve to further generalize the great bridge forming tendency of the coordinated mercaptide group, revealing that triple, as well as double, bridges may be formed. It is likely that single mercaptide bridges are actually the least abundant. Continuing investigations in these Laboratories suggest that forced trans planar structures, such as that of Ni(R2NCH2CH2S)2, are not conducive to bridge formation. It is expected that the several investigations on the complexes of mercaptoamines presently in progress in these Laboratories will further delineate the factors determining the tendency of the mercaptide function to coordinate to one and two metal ions. It is not unlikely that this function might bind three metal ions together under particularly favorable conditions.
Experimental The determinations of conductances and magnetic moments were conducted as reported previously.lV2 Infrared spectra were routinely utilized for control purposes. Analyses were performed by Galbraith Microanalytical Laboratories. Tris-(p-mercaptoethy1amine)-cobalt(111) [ Co(NH2CH2CH2S)3].-Five g. of potassium ethylenediaminetetracetatocobaltate( 111) dihydrate (0.0109 mole) was dissolved in 75 ml. of water. To this solution were added 3.79 g. of 6-mercaptoethylamine hydrochloride (0.0334 mole) and 2.62 g. of sodium hydroxide (0.0654 mole) in 75 ml. of water. The resulting solution was stirred and heated gently during the displacement reaction which ensued. After several minutes a heavy blue flocculent solid started to precipitate from solution. Stirring was continued for an additional period of approximately0.5 hr., and the solid was collected by filtration. Because of its flocculent nature the solid produced a thick matte on the filter funnel making it extremely difficult to wash the substance free of solution, Therefore, the partially dry solid was transferred to a flask and washed by decantation with 100-ml. portions of water ( 8 ) P. E. Figgins and D. H. Busch, J . A m . Chem. Soc., 82, 820 (1960). (9) C. Root and D. H. Busch, unpublished results.
Inorganic Chemistry until the wash water was nearly colorless. The solid was filtered again and washed twice with 25-ml. portions of water and twice with 25-1111. portions of absolute ethanol; yield, 2.1 g. A small amount of the complex crystallized from the filtrate on standing for several hours (0.18 g.); total yield, 2.28 g. (91.6%). A n d . Calcd. for JCo(NH2CHzCHzS),]: C, 25.08; H, 6.32; iY, 14.62; S, 33.47. Found: C,25.03; H, 6.02; N, 14.63; S, 33.13. Hexakis-( p-mercaptoethy1amine)-tricobalt(111)Bromide, [Co { Co(NH2CHzCH2S)3) 2]Br3. Method I.-Bromopentamminecobalt(II1) bromide, [Co(NH3)SBr]Br2,(3.96 g., 0.010 mole) was added to a suspension of 4.74 g. of tris(0-mercaptoethy1amine)-cobalt(II1)in 400 ml. of water. The resulting suspension was heated and stirred for approximately 1 hr., during which time much of the solid dissolved forming a deep red-brown solution. The solubilization was accompanied by the evolution of ammonia gas. A small amount of undissolved [Co(NHzCHZCHZS)~] was removed by filtration and washed free of the solution with a small quantity of water. The solution was filtered again a t room temperature and concentrated in z)acuo to approximately 250 ml. The filtrate was warmed and a warm saturated solution of potassium bromide (3.5 9.) was added slowly. The solution was allowed to stand in the cold for a few hours. The dark crystals (red-brown when viewed under a microscope) were filtered, washed twice with cold absolute ethanol, and dried in vacuo over P4010with continuous pumping; yield, 2.91 g. Concentration of the highly colored filtrate to one-third its volume produced 3.54 g. of crystalline product upon cooling for several hours; total yield, 6.45 g. (73.8%). Anal. Calcd. for [CO{CO(NHZCH~CH~S)~}~]B~~: C, 16.50; H , 4.16; N, 9.62; S, 22.03; Br, 27.45. Found: C, 16.77; H , 4.39; N, 9.78; S, 21.53; Br, 27.39. Hexakis-(p-mercaptoethy1amine)-tricobalt(II1)Bromide. Method 2.-The substitution of cobalt( 11) chloride 6hydrate in place of the brornopentamminecobalt(111) bromide resulted in the formation of a deep red solution from which [Co(Co(NH&HpCH2S),\z]Br3 was isolated by the addition of a saturated solution of potassium bromide in water; yield, 65.9%. Anal. Calcd. for [Co(Co(NHpC H Z C H ~ S ) ~ } ~C,] B 16.50; ~ ~ : H , 4.16; IT,9.62; S, 22.03; Br, 27.45. Found: C, 16.61; H,4.58; N,9.73; S,22.47; Br, 27.38. The corresponding chloride salt is much more soluble in water and in absolute ethanol and could not be isolated in an acceptable state of purity. Hexakis-(p-mercaptoethylamine) - dicobalt(111)-nickel(11) Bromide, [ Ni { Co(NH2CHzCHzS)3 ] BrB.-This compound was prepared by Method 2 employed for the corresponding cobalt( 111) adduct of [eo( S H ~ C H Z C H ~ and S)~] isolated in the form of the bromide salt by the addition of a saturated solution of potassium bromide to a warm solution of the chloride salt. The bromide of nickel(I1) is slightly less soluble than the cobalt( 111) adduct and larger crystals are formed if the solution is kept warm during the addition of potassium bromide; yield, 80.6%. Anal. Calcd. for [N~{CO(NHZCHZCH~S)~},]B~~: C, 18.16; H, 4.58; S , 10.60; S, 24.25; Br, 20.15. Found: C, 18.34; H , 4.72; N, 10.34; S, 23.97; Br, 19.86. Aqueous solutions of the cobalt( 111) and nickel( 11: adducts of [ Co( h"zCH2CH2S)3]undergo no apparent decomposition upon standing for a few days; however, the
STRUCTURE OF CHELIDAMIC ACID 887
Vol. 1, No. 4, November, 1962 addition of aqueous ammonia promotes the separation of [Co( NH&Hd2H*S)a] from solution.
Acknowledgment.-The
financial support of
the National Institutes of Health is gratefully acknowledged. The help of Mr. Charles A. Root, who confirmed the synthetic procedures, is sincerely appreciated.
CONTRIBUTION FROM THE DEPARTMENT OF CHEMISTRY. UNIVERSITY OF ARIZONA, TUCSON,ARIZONA
The Influence of Metal Chelation on the Structure of Chelidamic Acid BY SASWATI P. BAG, QUINTUS FERNANDO, AND HENRY FREISER
Received April 9, 1962 The acid dissociation constants of 2,6-dicarboxy-4-hydroxypyridine(chelidamic acid) have been determined potentiometrically in water at 25". Metal chelation was found to have an acid-strengthening effect on the 4hydroxy group in chelidamic acid. The extent of this effect varied with the metal ion that was chelated. The pK, of the hydroxy group in the metal chelate was found to increase in the order: Cu(I1) < Co(I1) < Zn(I1) < Ni(I1) < Mn(I1).
has been centered largely around the effects that the ligand molecule, or various modifications of it, has on the chemistry of the metal ion. On the other hand the effect of metal ion chelation on the properties of the organic moiety should be of great interest, especially to biochemists. It is well known that 2-hydroxypyridine and 4hydroxypyridine in solution exist largely in their tautomeric forms as 2-pyridone and 4-pyridone, respectively, due to keto-enol tautomerism. In the present study, chelidamic acid, a substituted 4-pyridone with chelate-forming groups, was selected to determine whether chelation had any effect on this keto-enol tautomerism. 0
HOOCQCOOH
= & HOOC
COOH
Enol form
Keto form
Experimental Purification of Chelidamic Acid and 2,B-Pyridinedicarboxylic Acid.-Chelidamic acid, obtained from K and K Laboratories, Inc., New York, N. Y., was purified by the following procedure: the crude acid was dissolved in 1 M NHs and warmed with .animal charcoal, the solution was filtered, and the filtrate was neutralized with dilute H N 0 3 to precipitate a pale yellow solid. This solid was redissolved in warm dilute "03 and the solution was cooled and diluted, to yield a white precipitate. This precipitate was washed repeatedly with ice-cold water and finally re-
colorless, m.p. 250"; lit.1 m.p. 248'. The equivalent weight of the compound was determined by a potentiometric titration (calcd., 91.56; found, 92.20). 2,6-Pyridinedicarboxylic acid was obtained from K and K Laboratories, Inc., New York, N. Y. The compound, after recrystallizing several times from hot water and finally from ethanol, was colorless and melted with decomposition a t 252' (lit.* 255'). The equivalent weight of the compound was determined by a potentiometric titration (calcd., 83.56; found, 83.80). Acid Dissociation Constants.-The acid dissociation constants of chelidamic acid were determined potentiometrically as follows: a weighed quantity of the acid was dissolved in 100 ml. of water and titrated in a waterjacketed vessel at 25 f 0.1" in an atmosphere of nitrogen with a solution of carbonate-free NaOH. A Beckman Model G pH meter, equipped with a glass-saturated calomel electrode pair and calibrated with buffer solutions at pH 4.00 and 7.00, was used for all p H measurements. The acid dissociation constants of chelidamic acid also were determined spectrophotometrically by measuring the absorbances of a series of solutions of varying pH containing 4 X IO-' M chelidamic acid. The ionic strength of each solution was maintained at 0.1 by the addition of NaC104. The p H values of solutions in the extreme ranges ( < 2 or > 11) were adjusted with perchloric acid or carbonate-free sodium hydroxide. I n the intermediate ranges of pH, buffer solutions were used. The buffer components were CHICOSH, Na[O2CCH3],NaB02, K ~ H P O IKHIPO,, , and "3.
The ultraviolet spectra of all solutions were obtained with a Beckman Model DB or Cary Model 11 recording (1) E. Riegel and M.C. Reinhard, J . Am. Chem. SOG.,48, 1334 (1928). (2) M. Henze, Ber., 67, 751 (1934).
